Phase Tracking in Communications Systems

ABSTRACT

The present invention includes a method of determining a phase estimate for an input signal having pilot symbols. The method includes receiving a plurality of pilot symbols, and then multiplying two or more pilot symbol slots by corresponding correlator coefficients to correct a phase estimate of the input signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of allowed U.S. application Ser. No.13/604,423, filed Sep. 5, 2012, which is a continuation of U.S.application Ser. No. 11/586,668, filed Oct. 26, 2006 and issued as U.S.Pat. No. 8,265,217 on Sep. 11, 2012, which claims benefit to U.S.Provisional Application No. 60/730,376, filed Oct. 27, 2005, all ofwhich are incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to phase tracking incommunication systems that use pilot symbols, including for example,satellite communication systems.

2. Background

For a communication system with a very low signal-to-noise ratio (SNR)such as satellite system, known pilot symbols are inserted periodicallyin the data frame for the purpose of phase tracking. The pilot symbolsare used to correct for Doppler effects, as well as phase noise, thatcause phase tracking errors between the incoming carrier signal and alocal oscillator within the satellite receiver. Specifically, the pilotsymbols are used to estimate the phase of the incoming signal (hereincalled the phase estimate). For example, the phase estimate can be anestimate of the phase delay introduced in the carrier signal by theDoppler effects.

For example, FIG. 1 illustrates a data steam with pilot symbol slots 102a-d that are inserted in the data stream where each symbol slot can haveone or more pilot symbols. The pilot symbols are used to generate thephase estimates, and the phase estimates are used to align the phase ofthe incoming signal with the phase of the local oscillator in thedigital receiver portion of a satellite receiver. This alignment processis known as phase tracking or phase correction. In order to minimize theoverhead for pilot symbols, the number of pilot symbols in each slot iskept as small as possible.

Conventional communication systems use phase estimates that are based onone slot of pilot symbols to adjust the phase of the receiver's localoscillator. However, a problem can occur in low SNR communicationsystems. In these low SNR systems, phase estimates that are based on oneslot of pilot symbols may not be sufficient to accommodate the requiredsystem performance.

What is needed, therefore, is an apparatus and method of determining thephase estimate of the incoming data signal using multiple pilot symbolslots and then using those phase estimates for the purpose of phasetracking

BRIEF SUMMARY OF THE INVENTION

The present invention includes a method of determining a phase estimatefor an input signal having pilot symbols. The method includes receivinga plurality of pilot symbols, and then multiplying two or more adjacentpilot symbol slots by corresponding correlator coefficients to produce aplurality of correlator outputs that are combined to produce phaseestimate of the input signal.

The present invention provides a technique for providing phaseestimations that use pilot symbols from multiple slots, spaced apart toachieve a better estimate than those of prior art systems that use onlyone slot. More specifically, the present invention considers one or moreadjacent (e.g., neighboring) pilot symbol slots to perform the phaseestimation that is used for phase tracking. Traditional systems, asnoted above, typically use only one slot of pilot symbols to perform thephase estimation that is used for the phase tracking. By considering oneor more neighboring pilot symbol slots, the present invention decreasesthe phase estimation variance in communication systems where the SNR isseverely compromised, thereby enhancing the system's phase trackingcapabilities.

Further features and advantages of the present invention, as well as thestructure and operation of various embodiments of the present invention,are described in detail below with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described with reference to the accompanyingdrawings. In the drawings, like reference numbers indicate identical orfunctionally similar elements. Additionally, the left-most digit(s) of areference number identifies the drawing in which the reference numberfirst appears.

FIG. 1 is an illustration of a data stream having pilot symbols embeddedtherein.

FIG. 2 is a block diagram illustration of a phase estimation andtracking system that receives incoming data, phase corrects the incomingdata, and produces output data in accordance with the present invention.

FIG. 3 is a block diagram illustration of a conventional phase estimatorbased on the determination of a phase estimate using one pilot symbolslot.

FIG. 4 is a block diagram illustration of a phase estimator that weightsphase estimates from multiple pilot symbol slots over time according toembodiments of the invention.

FIG. 5 is a flow chart of an exemplary method of practicing anembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the present invention refers tothe accompanying drawings that illustrate exemplary embodimentsconsistent with this invention. Other embodiments are possible, andmodifications may be made to the embodiments within the spirit and scopeof the invention. Therefore, the following detailed description is notmeant to limit the invention. Rather, the scope of the invention isdefined by the appending claims.

It will be apparent to one skilled in the art that the presentinvention, as described below, may be implemented in many differentembodiments. Any actual software code implementing the present inventionis not limiting of the present invention. Thus, the operational behaviorof the present invention will be described with the understanding thatmodifications and variations of the embodiments are possible, given thelevel of detail presented herein.

FIG. 1 is a block diagram illustration of a data stream 100 in the timedomain having embedded pilot symbol slots 102 (a-d) to provide for phaseestimation and correction of phase noise. Each slot consists of one ormore known symbols, also known as pilot symbols. Segments (d) within thedata stream 100 are representative of payload data. In the exemplaryembodiment of FIG. 1, a spacing between pilot symbol slots, for example,between the symbols 102(a) and 102(b) is 1440 symbols. However, othersystems could use other spacing values that would also be within thespirit and scope of the present invention.

FIG. 2 is a block diagram illustration of an architecture of a phaseestimation and correction system 200 that uses pilot symbols to providephase estimates. Stated another way, the tracking scheme of the system200 is used to align the phase of the incoming data with the localoscillator phase in the satellite receiver. The system 200 receives anincoming input data stream 202, phase corrects the incoming data, andproduces output data 218. The system 200 includes a delay buffer 204, amultiplier 206, a phase estimate module 208, a summer 210, a processingmodule 212, a delay 216, and a pilot flag module 213 a.

In FIG. 2, the pilot flag module 213 a produces an enable phase estimatesignal 213 b to enable the phase estimate module 208. The pilot enablesignal 213 b is generated when the pilot flag module 213 a senses thepresence of a pilot symbol slot within the input data stream 202, suchas any one of the pilot symbol slots 102 (a-d), shown in FIG. 1. Thatis, the phase estimate module 208 is only activated to estimate phasewhen the module 208 is triggered by the enable 213 b. The phase estimatemodule 208 generates a current phase estimate 214 that is used to alignthe input data stream 202 with the phase of the local oscillator.

During operation of the system 200, the input data stream 202 is storedin the delay buffer 204. The pilot symbols within the slots 102 (a-d)are stripped from the input data stream 202 and forwarded as a pilotsymbol signal 203 to the phase estimate module 208, to produce thecurrent phase estimate 214 as an output.

After the phase of the pilot symbol signal 203 is estimated within thephase estimate module 208, the current phase estimate value 214 isoutput from the phase estimate module 208. For example, the phase can beestimated for the pilot symbol slot 102(b) of the data stream 100. Aprevious phase estimate value (e.g., previous phase estimate for thepilot symbol slot 102(a)), is stored within the delay module 216. Theprevious phase estimate value for 102(a) is then compared with thecurrent phase estimate value 214 (representative of the phase of 102(b))within the summer 210 to produce a difference phase estimate value 211.

The difference phase estimate value 211, which represents a phasedifference between the pilot symbol slots 102(a) and 102(b), is latershaped as a linear phase ramp. This ramp is then converted to a complexsinusoidal value that is applied to the data stored within the delaybuffer 204. This aspect of the present invention is discussed in greaterdetail below.

The processing module 212 is configured to produce a complex sinusoidalsignal that is based on a linear phase ramp. The complex sinusoidalsignal is applied to the data output from the delay buffer 204. Theprocessing module 212 includes a scalar 220 and a digital directfrequency synthesizer (DDFS) module 222. The scalar 220 divides thedifference phase estimate value 211 by the number of data symbols (e.g.,1440 symbols), between the pilot symbols 102(a) and 102(b). The DDFS 222then produces the linear phase ramp based on the divided differencephase estimate. The ramp start and end points correspond to the phaseestimate values for pilot symbol slots 102(a) and 102(b), respectivelyThe linear phase ramp is produced based upon an output from the scalar220 in accordance with techniques well known to those of skill in theart.

The linear phase ramp is produced by the DDFS 222, which then convertsthe phase ramp to a complex sinusoidal value. More specifically, thereal part of the DDFS output corresponds to the sine of the phase ramp,and the imaginary part of the DDFS output corresponds to the cosine ofthe phase ramp. The output from the DDFS 222 is then used to correct thephase of the input data stream 202 that is stored within the delaybuffer 204. More specifically, the DDFS outputs are multiplied with theoutput from the delay buffer 204 to produce phase corrected output data218. In this manner, a phase correction is applied to data from the datastream 100 that lies between the pilot symbol slots 102(a) and 102(b),using phase estimates based on the pilot symbol slots 102(a) and 102(b).

FIG. 3 illustrates a conventional phase estimator 300 configured toproduce a phase estimate 314. The conventional phase estimator 300includes a data register 302, multipliers 304 a-c, a pilot symbolcorrelator coefficient generator 305, and a complex number to phaseconversion unit 308.

During operation, the pilot symbols 203 are shifted into the dataregister 302.

In the phase estimator 300, the number of multipliers within the dataregister 302 corresponds to the number of pilot symbols within the datastream 100. The multipliers 304 multiply corresponding pilot data bypilot symbol coefficients that are generated by the correlatorcoefficient generator 305, to retrieve the actual pilot symbol data. Byway of example, the coefficients can be conjugates of the correspondingpilot symbols.

The resulting outputs from the multipliers 304 a-c are added togetherusing the adder 306 to produce a correlator output signal 307. Thecorrelator output signal 307 is processed by the complex number to phaseconversion unit 308 to convert the correlator output signal 307 to aphase, which is the phase estimate 314.

The phase estimate is calculated using a coherent correlator followed bya complex number to phase conversion unit, as shown in FIG. 3. Given asequence of pilot symbols P, the correlator coefficient vector C can beexpressed as follows:

P={e ^(jφ) ⁰ ,e ^(jφ) ¹ ,e ^(jφ) ² ,e ^(jφ) ³ , . . . }

C={e ^(−jφ) ⁰ ,e ^(−jφ) ¹ ,e ^(−jφ) ² ,e ^(−jφ) ³ , . . . }

The phase estimate variance is limited by the number of symbols in apilot slot. Since the estimate variance determines the overall receiverperformance, it is desirable to make the phase estimate variancesmaller. A smaller phase estimate variance can be achieved using longerpilot slots, but that costs system bandwidth. On the other hand, thephase change in a receiver is gradual when the system is locked. Usingthis property, a weighted function can be used to smooth out the currentphase estimate. An implementation of this weighted phase estimate isshown in FIG. 4. In the implementation of FIG. 4, an earlier phaseestimate and a later phase estimate are used. This architecture canreduce the estimation variance by a factor of three.

In general, the variance of phase estimates in pilot symbol basedcommunication systems, such as the estimator 300, is dependent on thenumber of symbols used to generate the estimate. The phase estimate 314is only based on one pilot symbol slot, using 36 symbols. Since thesystem 300, which does not have the ability to change the number ofsymbols in a pilot symbol slot, uses only one pilot symbol slot tocreate its estimate, it may not be robust enough in a high noiseenvironment. A greater number of symbols (e.g., 72, 108, of higher)would provide an estimate with lower variance, which would ultimatelytranslate to output payload data with a high signal to noise ratio.

FIG. 4 is a block diagram illustration of a phase estimator 400constructed in accordance with an embodiment of the present invention.The phase estimator 400 of FIG. 4 weighs two or more phase estimatesover time in order to improve the phase estimate in a noisy environment.Based upon simulations and test measurements illustrating that a greaternumbers of pilot symbols reduces the phase estimation variance, thephase estimator 400 is configured to change the number of symbols usedin the estimate. So, for example, instead of being restricted to usingonly the current pilot symbol slot to create the estimate, neighboringpilot symbol slots can also be used.

Phase estimator 400 includes the data register 302, the multipliers 304,the coefficient generator 305, and the adder 306. However, the phaseestimator 400 also includes a weight coefficient generator 402, delays404 a and 404 b, multipliers 406 a-c, the adder 408, and a complexnumber to phase converter 401. Note that there could be more than twodelays 404 depending the number of phase estimates that are averaged,which is determined by the specific embodiment.

In addition to the functions discussed above for the phase estimator300, the phase estimator 400 also weights and averages one or more phaseestimates over time (past, present, and future). The correlator output307 is delayed by first and second delays 404 a and 404 b. Accordingly,the multipliers 406 a-c process a past, a present, and a futurecorrelator output 307. By doing so, the phase estimator 400 averagespast, present, and future phase estimates to produce a final phaseestimate 414, where the average is performed in a weighted fashion.

Specifically, the multiplier 406 a multiplies the correlator output 307by a first weighting coefficient, to produce a first weighted correlatoroutput 407 a. The weight coefficient generator 402 produces a set ofweighting coefficients based upon user calculations. That is, thecoefficients are pre-calculated based upon characteristics of thecommunication system.

The multiplier 406 b multiplies the correlator output 307 b by a secondweighting coefficient, to produce a second correlator output 407 b. Themultiplier 406 c multiplies the correlator output 307 c by a thirdweighting coefficient, to produce a third correlator output 407 c. Theweighted correlator outputs 407 a-c are summed by the adder 408, toproduce a combined correlator output 411. The complex number to phaseconverter 401 converts the combined correlator output 411 to a phase,which provides the weighted phase estimate output 414 that is used forthe phase estimate 214 in FIG. 2.

Note that any number of correlator outputs 307 can be weighted andaveraged. Thus, the present invention is not limited to three as shown.

Regarding the weighting coefficients, in a slow moving channel, an equalweighting can be used: ⅓ (past), ⅓ (present), ⅓ (future). In oneembodiment, a weighting of ¼ (past), ½ (present), ¼ (future).

FIG. 5 is a flow chart of an exemplary method 500 of practicing anembodiment of the present invention. In the method 500, a plurality ofpilot symbol slots is received in a step 502. In step 504, two or moreadjacent pilot symbol slots are multiplied by corresponding correlatorcoefficients to produce a plurality of correlator outputs to correct aphase estimate of the input signal. The plurality of correlator outputsare averaged. The result of the averaging is then converted from acomplex representation to a phase representation.

CONCLUSION

Example embodiments of the methods, systems, and components of thepresent invention have been described herein. As noted elsewhere, theseexample embodiments have been described for illustrative purposes only,and are not limiting. Other embodiments are possible and are covered bythe invention. Such other embodiments will be apparent to personsskilled in the relevant art(s) based on the teachings contained herein.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

What is claimed is:
 1. A method comprising: receiving a first set ofpilot symbols and a second set of pilot symbols in a data stream,wherein the data stream comprises data symbols positioned between thefirst set and second set of pilot symbols, and wherein the first andsecond set of pilot symbols each comprise one or more pilot symbols;generating a first phase estimate and a second phase estimate based onthe first set of pilot symbols and the second set of pilot symbolsrespectively; determining a phase difference between the second phaseestimate and the first phase estimate; and correcting phases associatedwith the data symbols in the data stream based on the phase difference,wherein the receiving, generating, determining and correcting areperformed by one or more circuits.
 2. The method of claim 1, whereindetermining the phase difference includes: delaying the first phaseestimate by a delay unit to form a delayed first phase estimate; andsubtracting the delayed first phase estimate from the second phaseestimate to form the phase difference.
 3. The method of claim 1, whereingenerating the first phase estimate includes: forming a first correlatoroutput by multiplying the first set of pilot symbols with respectivepilot symbol coefficients; forming a second correlator output bymultiplying the second set of pilot symbols with respective pilot symbolcoefficients; delaying the first correlator output by a delay unit tooutput a delayed first correlator output; and forming a weighted sum ofthe delayed first correlator output and the second correlator output togenerate the first phase estimate.
 4. The method of claim 3, whereinforming the weighted sum of the delayed first correlator output and thesecond correlator output includes: multiplying the delayed firstcorrelator output and the second correlator output with a firstweighting coefficient and a second weighting coefficient respectively.5. The method of claim 4, wherein the first weighting coefficient andthe second weighting coefficient are based on characteristics of acommunication system that uses the data stream.
 6. The method of claim3, wherein the pilot symbol coefficients comprise conjugates ofcorresponding pilot symbols.
 7. The method of claim 1, where generatingthe first phase estimate and the second phase estimate includestriggering by sensing a presence of the first set of pilot symbols andthe second set of pilot symbols respectively.
 8. The method of claim 1,wherein correcting phases of the data symbols includes: generating aphase ramp representative of respective beginning and ending pointsassociated with the first and second pilot symbols; converting the phaseramp to a complex sinusoidal value; and multiplying the complexsinusoidal value with the data symbols to correct phases of the datasymbols.
 9. The method of claim 1, wherein correcting phases of the datasymbols includes: forming a complex sinusoid using a scalar value and adirect digital frequency synthesizer.
 10. The method of claim 9, whereinthe scalar value is derived from a number of the data symbols.
 11. Asystem comprising: one or more circuits configured to: receive a firstset of pilot symbols and a second set of pilot symbols in a data stream,wherein the data stream comprises data symbols positioned between thefirst set and second set of pilot symbols, and wherein the first andsecond set of pilot symbols each comprise one or more pilot symbols;generate a first phase estimate and a second phase estimate based on thefirst set of pilot symbols and the second set of pilot symbolsrespectively; determine a phase difference between the second phaseestimate and the first phase estimate; and correct phases associatedwith the data symbols in the data stream based on the phase difference.12. The system of claim 11, wherein the one or more circuits are furtherconfigured to determine the phase difference by: delaying the firstphase estimate by a delay unit to form a delayed first phase estimate;and subtracting the delayed first phase estimate from the second phaseestimate to form the phase difference.
 13. The system of claim 11,wherein the one or more circuits are further configured to generate thefirst phase estimate by: forming a first correlator output bymultiplying the first set of pilot symbols with respective pilot symbolcoefficients; forming a second correlator output by multiplying thesecond set of pilot symbols with respective pilot symbol coefficients;delaying the first correlator output by a delay unit to output a delayedfirst correlator output; and forming a weighted sum of the delayed firstcorrelator output and the second correlator output to generate the firstphase estimate.
 14. The system of claim 13, wherein the one or morecircuits are further configured to form the weighted sum of the delayedfirst correlator output and the second correlator output by: multiplyingthe delayed first correlator output and the second correlator outputwith a first weighting coefficient and a second weighting coefficientrespectively.
 15. The system of claim 14, wherein the first weightingcoefficient and the second weighting coefficient are based oncharacteristics of a communication system that uses the data stream. 16.The system of claim 13, wherein the pilot symbol coefficients compriseconjugates of corresponding pilot symbols.
 17. The system of claim 11,wherein the one or more circuits are further configured to generate thefirst phase estimate and the second phase estimate by triggering throughsensing a presence of the first set of pilot symbols and the second setof pilot symbols respectively.
 18. The system of claim 11, wherein theone or more circuits are further configured to correct phases of thedata symbols by: generating a phase ramp representative of respectivebeginning and ending points associated with the first and second pilotsymbols; converting the phase ramp to a complex sinusoidal value; andmultiplying the complex sinusoidal value with the data symbols tocorrect phases of the data symbols.
 19. The system of claim 11, whereinthe one or more circuits are further configured to correct phases of thedata symbols by: forming a complex sinusoid using a scalar value and adirect digital frequency synthesizer.
 20. The system of claim 19,wherein the scalar value is derived from a number of the data symbols.